Monolithic reconfigurable optical multiplexer systems and methods

ABSTRACT

A silicon demultiplexer, a plurality of silicon switches and a silicon multiplexer are monolithically integrated on a single silicon chip. In embodiments, the silicon demultiplexer and the silicon multiplexer each comprise a diffraction grating. In other embodiments, the silicon demultiplexer and the silicon multiplexer each comprise an arrayed waveguide grating. In various exemplary embodiments, the silicon optical switches comprise optical switches, micromachined torsion mirrors, electrostatic micromirrors, and/or tilting micromirrors. In use, an optical signal comprising a multiplexed data stream is input into the monolithic reconfigurable optical multiplexer. An optical signal that comprises a modified multiplexed data stream may be output. In an optical communications system, the silicon demultiplexer communicates with an input optical fiber, the plurality of silicon optical switches communicate between the silicon demultiplexer and the silicon multiplexer, and the silicon multiplexer communicates with an output optical fiber. In various embodiments, the optical switches are fabricated to be self-aligned.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to optical micromachined ormicroelectromechanical system based multiplexers and multiplexingmethods.

2. Description of Related Art

Multiplexers are generally well-known. For example, an opticalmultiplexer/demultiplexer comprising an array of optical waveguides isdescribed in U.S. Pat. No. 5,002,350 to Dragone. For opticalapplications, an optical add/drop multiplexer receives an input opticalsignal with many optical channels at different wavelengths from a singleoptical fiber. The optical signal is demultiplexed into separate opticalchannels based on their wavelengths. Once demultiplexed, each of theseparate optical channels can either pass through the optical add/dropmultiplexer to a multiplexer or be dropped. For any channel that isdropped, a new signal can be added to utilize that channel. The passedand added channels are remultiplexed into an output optical signal sentout on a single optical fiber.

Current optical add/drop multiplexers are assembled from discretecomponents including demultiplexers, switches and multiplexers. Typicalmultiplexers and demultiplexers include diffraction gratings in freespace optics and arrayed waveguide gratings for guided wave optics.Optical switches are used for dropping, adding and passing channels.

SUMMARY OF THE INVENTION

The systems and methods of this invention provide high quality opticalmultiplexing of an optical signal with improved performance.

The systems and methods of this invention separately provide opticalmultiplexers with improved manufacturability and reduced manufacturingcosts.

The systems and methods of this invention separately provide opticalmultiplexers with reduced size and weight.

The systems and methods of this invention separately provide opticalmultiplexers with latching switches.

The systems and methods of this invention separately provide monolithicintegration of optical multiplexers and demultiplexers with opticalswitches.

The systems and methods of this invention separately and independentlyprovide a micro-optical device having an aligned waveguide switch.

According to various exemplary embodiments of the systems and methods ofthis invention, a silicon demultiplexer, a plurality of silicon switchesand a silicon multiplexer are monolithically integrated on a singlesilicon chip. In embodiments, the silicon demultiplexer and the siliconmultiplexer each comprise a diffraction grating. In other embodiments,the silicon demultiplexer and the silicon multiplexer each comprise anarrayed waveguide grating. In various exemplary embodiments, the siliconoptical switches comprise 1×2 or 2×2 or m×n optical switches, opticalchangeover switches, micromachined torsion mirrors, electrostatic,magnetostatic, piezoelectric or thermal micromirrors, and/or tiltingmicromirrors.

According to various exemplary embodiments of the systems and methods ofthis invention, an optical signal is input into a monolithicreconfigurable optical multiplexer. The input optical signal comprises adata stream. The optical multiplexer includes at least one silicondemultiplexer, a plurality of silicon optical switches and at least onesilicon multiplexer integrated on a single silicon chip. In embodiments,an optical signal is output that comprises a modified data stream.

According to various exemplary embodiments of the systems and methods ofthis invention, an optical communications system comprises an inputoptical fiber, a silicon demultiplexer communicating with the inputoptical fiber, a silicon multiplexer, a plurality of silicon opticalswitches communicating between the silicon demultiplexer and the siliconmultiplexer and an output optical fiber communicating with the siliconmultiplexer. The silicon demultiplexer, optical switches and multiplexerare monolithically integrated on a single silicon chip.

These and other features and advantages of this invention are describedin, or are apparent from, the following detailed description of variousexemplary embodiments of the systems and methods according to thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of the systems and methods of thisinvention described in detail below, with reference to the attacheddrawing figures, in which:

FIG. 1 is a schematic representation of a conventional reconfigurableoptical add/drop multiplexer;

FIG. 2 is schematic representation of an exemplary embodiment of areconfigurable optical multiplexer according to this invention;

FIG. 3 is a cross-sectional view of the exemplary embodiment of FIG. 2as incorporated into an optical communications system;

FIG. 4 is an exemplary embodiment of a switch for a reconfigurableoptical multiplexer according to this invention;

FIGS. 5-10 show a first exemplary embodiment of a self-aligned waveguideswitch according to this invention;

FIGS. 11-18 illustrate various stages of a first exemplary embodiment ofa fabrication process for a self-aligned waveguide switch according tothis invention;

FIGS. 19-24 illustrate various stages of a second exemplary embodimentof a fabrication process for a self-aligned waveguide switch accordingto this invention;

FIGS. 25-26 illustrate a modification of the second exemplary embodimentof FIGS. 19-24 according to this invention;

FIGS. 27-57 illustrate a more detailed exemplary embodiment of afabrication process for a self-aligned waveguide switch according tothis invention; and

FIGS. 58-68 illustrate a modification of the more detailed exemplaryembodiment of FIGS. 27-57 according to this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

While the invention is described hereafter with reference to an add/dropmultiplexer, it should be understood that this invention is not strictlylimited to adding and/or dropping signals. Rather, any device thatallows modification of a signal via multiplexing after demultiplexing iscontemplated by this invention.

The systems and methods of this invention provide monolithic integrationof optical multiplexers and demultiplexers with optical switches on asilicon chip for use as a reconfigurable optical multiplexer. Thus, areconfigurable optical multiplexer according to various embodiments ofthis invention comprises a silicon demultiplexer, a plurality of siliconswitches and a silicon multiplexer monolithically integrated on a singlesilicon chip. The monolithic integration of this invention can improvethe manufacturability of reconfigurable optical multiplexers, leading toreduced costs. Also, the monolithic integration of this inventionprovides a relatively compact optical multiplexer of significantlyreduced size and weight. Further, reconfigurable optical multiplexersaccording to this invention can provide higher quality opticalmultiplexing of an optical signal with improved performance.

In various exemplary embodiments, the silicon demultiplexer and thesilicon multiplexer each comprise a diffraction grating. In otherexemplary embodiments, the silicon demultiplexer and the siliconmultiplexer each comprise an arrayed waveguide grating. The siliconoptical switches may comprise 1×2 or 2×2 or, in general, m×n opticalswitches, optical changeover switches, micromachined torsion mirrors,electrostatic, magnetostatic, piezoelectric or thermal micromirrors,and/or tilting micromirrors.

According to various exemplary embodiments, an optical signal is inputinto a monolithic reconfigurable optical multiplexer of this invention.The input optical signal may comprise a wavelength division multiplexed(WDM) data stream. The input optical signal is demultiplexed intoseparate channels according to wavelengths of light in the signal usingthe demultiplexer. Each channel is either passed through or dropped outusing the optical switches. For each channel that is dropped out, a newdata stream at the same wavelength may be added to utilize that channel.The channels are then multiplexed back together as an output opticalsignal using the multiplexer. The output optical signal may comprise amodified data stream, depending on the dropping/adding, or othermodification, of channels.

A monolithic reconfigurable optical multiplexer according to thisinvention may be incorporated into an optical communications system. Aninput optical fiber carrying a multiplexed optical signal maycommunicate with the demultiplexer and an output optical fiber maycommunicate with the multiplexer. The plurality of optical switches thencommunicate between the demultiplexer and the multiplexer to pass and/ormodify the optical signal. For example, the monolithic reconfigurableoptical multiplexer according to this invention may be incorporated intoa document device, such as a printer, a copier, a scanner, a facsimilemachine, a multi-function device or the like. Further, the monolithicreconfigurable optical multiplexer according to this invention may beincorporated into a distributed communication network. Thus, any systemor device that includes a distributed communication network iscontemplated by this invention.

According to various exemplary embodiments of this invention,micromachining and other microelectromechanical system basedmanufacturing techniques are used to fabricate a monolithicreconfigurable optical multiplexer. Such manufacturing technologies arerelatively advanced compared to other potential technologies, yieldingmore reliable results and greater flexibility.

In various exemplary embodiments, surface micromachining techniques areused to fabricate a monolithic reconfigurable optical multiplexer from asilicon on insulator (SOI) wafer as a starting substrate. In otherexemplary embodiments, surface micromachining techniques are used tofabricate a monolithic reconfigurable optical multiplexer from a firstwafer with a patterned semiconductor layer on at least one side and asecond wafer of single crystal silicon bonded to the semiconductor layeron the first wafer. The second wafer may also have a patternedsemiconductor layer on the side that is bonded to the semiconductorlayer on the first wafer.

A schematic representation of a conventional reconfigurable opticaladd/drop multiplexer 100 is shown in FIG. 1. The optical add/dropmultiplexer 100 receives an input optical signal 110 with many opticalchannels at different wavelengths from a single optical fiber. The inputoptical signal 110 is demultiplexed by a demultiplexer 120 into separateoptical channels 112 based on the wavelengths of the optical channels112. Once demultiplexed, each of the separate optical channels 112encounters one of a plurality of optical switches 130. The opticalswitches 130 can either pass or drop out the respective one of theoptical channels 112. For any of the optical channels 112 that aredropped, a new signal 114 can be added by the optical switches 130 toutilize that channel. Channels 116 that are passed or added by theoptical switches 130 are remultiplexed by a multiplexer 140 into anoutput optical signal 150 and output to a single optical fiber. Becausechannels may be dropped and added, the output optical signal 150 maycomprise a modified data stream as compared to the input optical signal110.

A schematic representation of a reconfigurable optical add/dropmultiplexer 200 according to this invention is shown in FIG. 2. As withthe conventional add/drop multiplexer 100, the optical add/dropmultiplexer 200 receives an input optical signal with many opticalchannels at different wavelengths from an input optical fiber 210. Thesignal from the input optical fiber 210 is demultiplexed by a silicondemultiplexer 220 into separate optical channels 212 based on thewavelengths of the optical channels 212. As shown in FIG. 2, the silicondemultiplexer 220 is an arrayed waveguide grating.

Once demultiplexed, each of the separate optical channels 212 of thesignal encounters one of a plurality of silicon optical switches 230.The silicon optical switches 230 can either pass or drop out therespective one of the optical channels 212 as a dropped signal 218. Forany of the optical channels 212 that are dropped, one or more newsignals 214 can be added by the silicon optical switches 230 to utilizethat channel. Channels 216 that are passed or added by the siliconoptical switches 230 are remultiplexed by a silicon multiplexer 240 intoan output optical signal that is output via an output optical fiber 250.Because channels may be dropped and added, the signal from the outputoptical fiber 250 may comprise a modified data stream as compared to thesignal from the input optical fiber 210.

As shown in FIG. 3, the reconfigurable optical add/drop multiplexer 200is formed by the silicon demultiplexer 220, the silicon optical switches230 and the silicon multiplexer 240 monolithically integrated on asingle silicon chip 202. The single silicon chip 202 may comprise asilicon on insulator (SOI) wafer 203 including a relatively thin singlecrystal silicon device layer 204 and an oxide layer 205. A relativelythick single crystal silicon handle layer 206 may be integrally bondedto the device layer 204 by the oxide layer 205 for structural support.Further, an auxiliary oxide or nitride layer 207 may be formed on anopposite side of the handling layer 206 for etching techniques. Thewafer 203 may be fabricated using any known or later developed siliconon insulator (SOI) techniques.

In the exemplary embodiment, the silicon demultiplexer 220, the siliconoptical switches 230 and the silicon multiplexer 240 are fabricated inthe device layer 204. One or more polysilicon layers (not shown) may beadded over the device layer 204 for fabrication of additional mechanicalelements, such as hinges, bridges, guides, anchors and the like, orelectrical elements, such as heaters, actuators or wires. Activeelectronic elements (not shown), such as electrical traces or logiccircuitry, may also be defined in the device layer 204.

An exemplary embodiment of one of the silicon optical switches 230 isshown in FIG. 4 as a waveguide switch or optical changeover switch. Theswitch 230 has a movable part 232 with a plurality of waveguides 234. Aninput waveguide 222 corresponding to one of the channels 212 from thesilicon demultiplexer 220 (shown in FIG. 2) and a waveguide 242 forcarrying the new signal 214 to be added are situated at one end of thewaveguides 234. Similarly, an output waveguide 224 corresponding to oneof the channels 216 to the silicon multiplexer 240 (shown in FIG. 2) anda waveguide 228 for dropping a signal are situated at the other end ofthe waveguides 234.

As indicated by the arrows in FIG. 4, the movable part 232 is movedtransversely by a pair of actuators 236. The actuators 236 may be of anysuitable type, such as, for example, thermal, electrostatic or magnetic.

The waveguides 234 are configured so that the transverse movement of themovable part 232 will switch between one of the waveguides 234connecting the input waveguide 222 to the output waveguide 224 and oneof the waveguides 234 connecting the waveguide 242 carrying the newsignal 214 to the output waveguide 224. To drop the signal of the inputwaveguide 222, one of the waveguides 234 can connect the input waveguide222 to the waveguide 228.

A suitable technique for fabricating the silicon demultiplexer 220, thesilicon optical switches 230 and the silicon multiplexer 240 in thedevice layer 204 is described in copending U.S. patent application Ser.No. 09/467,526 and U.S. Pat. Nos. 6,362,512 and 6,379,989, which areincorporated by reference in their entirety. Another suitable techniqueis described in copending U.S. patent application Ser. No. 09/718,017,which is incorporated by reference in its entirety.

The silicon demultiplexer 220 and the silicon multiplexer 240 may be anyknown or later developed multiplexer that is capable of fabrication insilicon. In particular, the silicon demultiplexer 220 and the siliconmultiplexer 240 may be diffraction gratings for free-space optics.Free-space optics may be preferred in applications where optical lossesare to be minimized. Such diffraction gratings may be fabricated usingthe techniques described in copending U.S. patent application Ser. No.09/467,184 and U.S. Pat. Nos. 6,249,346 and 6,399,405, which areincorporated by reference in their entirety.

When the silicon demultiplexer 220 and the silicon multiplexer 240 arediffraction gratings, any free-space optical switch capable of add/dropfunctionality and of fabrication in silicon may be used for the siliconoptical switches 230. Examples of known free-space optical switchesinclude those described in “Micro-Opto-Mechanical 2×2 Switch forSingle-Mode Fibers Based on Plasma-Etched Silicon Mirror andElectrostatic Actuation”, Cornel Marver et al., Journal of LightwaveTechnology, Vol. 17, No. 1, pp. 2-6 (1999); “Free-Space Fiber OpticSwitches Based on MEMS Vertical Torsional Mirrors”, Shi-Sheng Lee etal., Journal of Lightwave Technology, Vol. 17, No. 1, pp. 7-13 (1999);“Electrostatic Micro Torsion Mirrors for an Optical Switch Matrix”,Hiroshi Toshiyoshi et al., Journal of Microelectromechanical Systems,Vol. 5, No. 4, pp. 231-237 (1996); “Electromagnetic Torsion Mirrors forSelf-Aligned Fiber-Optic Cross-Connectors by Silicon Micromachining”,Hiroshi Toshiyoshi et al., IEEE Journal of Selected Topics in QuantumElectronics, Vol. 3, No. 1, pp. 10-17 (1999); “Free Space MicromachinedOptical Switches for Optical Networking”, L. Y. Lin et al., IEEE Journalof Selected Topics in Quantum Electronics, Vol. 3, No. 1, pp. 4-9(1999); “A Rotary Electrostatic Micromirror 1×8 Optical Switch”, A.Azzam Yasseen et al., IEEE Journal of Selected Topics in QuantumElectronics, Vol. 3, No. 1, pp. 26-32 (1999); and “Wavelength Add-DropSwitching Using Tilting Micromirrors”, Joseph E. Ford et al., Journal ofLightwave Technology, Vol. 17, No. 5, pp. 904-911 (1999), which areincorporated by reference in their entirety. Thus, the silicon opticalswitches 230 may be, for example, 1×2, 2×2 or m×n optical switches,micromachined torsion mirrors, electrostatic or magnetostaticmicromirrors, and/or tilting micromirrors and the like. For certainapplications, such as telecommunications, the silicon optical switches230 should be latching switches that retain their state when power islost.

Alternatively, the silicon demultiplexer 220 and the silicon multiplexer240 may be arrayed waveguide gratings for guided wave optics. Guidedwave optics allow simplified manufacture and avoid out-of-plane assemblythat may be required for free-space optical components. Thus, guidedwave optics may be preferred in applications where optical losses arenot a critical factor. Such arrayed waveguide gratings may be fabricatedusing any known or later developed techniques, such as those describedin “Advances in Silicon-on-Insulator Optoelectronics”, B. Jalali et al.,IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, No. 6,pp. 938-947 (1998), and “Arrayed waveguide grating demultiplexers insilicon-oninsulator”, M. R. T. Pearson et al., Proceedings of SPIESilicon-Based Monothic and Hybrid Optoelectronic Devices, Photonics WestMeeting, San Jose Calif., January 2000, which are incorporated byreference in their entirety.

When the silicon demultiplexer 220 and the silicon multiplexer 240 arearrayed waveguide gratings, any waveguide switch capable of add/dropfunctionality of fabrication in silicon may be used for the siliconoptical switches 230. Examples of known waveguide switches include thosedescribed in “Micro-opto mechanical switch integrated on silicon”, E.Ollier et al., Electronics Letters, Vol. 31, No. 23, pp. 2003-2005(1995); “Integrated electrostatic micro-switch for optical fibrenetworks driven by low voltage”, E. Ollier et al., Electronics Letters,Vol. 32, No. 21, pp. 2007-2009 (1996); “Micromechanical OpticalSwitching With Voltage Control Using SOI Moveable Integrated OpticalWaveguides”, Terry T. H. Eng et al., IEEE Photonics Technology Letters,Vol. 7, No. 11, pp. 1297-1299 (1995); and U.S. Pat. No. 5,002,354 toKoai, U.S. Pat. No. 5,261,015 to Glasheen and U.S. Pat. No. 5,612,815 toLabeye et al., which are incorporated by reference in their entirety.Thus, the silicon optical switches 230 may be, for example, micro-optomechanical switches, electrostatic or magnetostatic micro-switches,and/or integrated optical changeover switches and the like.

As noted above, the monolithic reconfigurable optical add/dropmultiplexer 200 according to this invention may be incorporated into anoptical communications system 20. As shown in FIG. 3, an input opticalfiber 22 carrying an optical signal is placed in communication with thesilicon demultiplexer 220 and an output optical fiber 24 is placed incommunication with the silicon multiplexer 240. The plurality of siliconoptical switches pass and/or modify the optical signal from the silicondemultiplexer 220, as described above, and send the optical signal tothe silicon multiplexer 240. Once remultiplexed, the optical signal,having been modified as desired, is passed to the output optical fiber24.

When the optical add/drop multiplexer 200 according to this invention isincorporated into the optical communications system 20, the inputoptical fiber 22 and the output optical fiber 24 need to be aligned withthe silicon demultiplexer 220 and the silicon multiplexer 240,respectively. This alignment may be accomplished by any known or laterdeveloped technique. For example, for free-space optics, the opticalfibers 22 and 24 may be aligned using a technique described in copendingU.S. Pat. No. 6,580,858, which is incorporated by reference in itsentirety.

FIGS. 5-10 show a first exemplary embodiment of a self-aligned waveguideswitch 330 for optical fiber communication that may be used in theoptical add/drop multiplexer of this invention. For various embodiments,the tolerance of misalignment between waveguides of the switch 330 isless than 0.5 microns to avoid unacceptable optical loss. The switch 330is self-aligned to implement a high precision optical system.

As shown in FIG. 5, the switch 330 includes a movable part 332 with aplurality of waveguides 334. A stationary input part 322 of the switch330 is in optical communication with, for example, the demultiplexer ofthe optical add/drop multiplexer and has a plurality of waveguides 324.A stationary output part 342 is in optical communication with, forexample, the multiplexer of the optical add/drop multiplexer and has aplurality of waveguides 344.

As shown in FIG. 6, a stop block 350 is anchored to a substrate 303 of,for example, the optical add/drop multiplexer. The stop block 350 isused to control the position of the movable part 332 of the switch 330by limiting the movement of the movable part 332. A set of offsets d1and d2 is defined between the waveguides 334 of the movable part 332 andthe waveguides 324 and 344 of the stationary parts 322 and 342. Asdescribed further below, the set of offsets d1 and d2 is defined byphotolithography before the movable part 332 is released from thesubstrate 303.

Also, one or more bumpers 352 may be constructed on the movable part 332of the switch 330. The same offsets d1 and d2 are used to locate thebumpers 352 such that the distance from the stop block 350 to an insideedge of one bumper 352 is d1 and the distance from the stop block 350 toan inside edge of the other bumper 352 is d2.

This arrangement provides two stable positions for the movable part 332of the switch 330. As shown in FIGS. 7 and 8, the movable part 332 movesto the left in direction of the arrow A until the stop block 350contacts one of the bumpers 352. In this position, the left waveguide334 of the movable part 332 is aligned with the left waveguides 324 and344 of the stationary parts 322 and 342. As shown in FIGS. 9 and 10, themovable part 332 moves to the right in direction of the arrow B untilthe stop block 350 contacts the other one of the bumpers 352. In thisposition, the left waveguide 334 of the movable part 332 is aligned withthe right waveguides 324 and 344 of the stationary parts 322 and 342.

An exemplary embodiment of a micromachining fabrication process for theself-aligned switch 330 is described with reference to FIGS. 11-18. Asshown in FIG. 12, the process begins with a silicon-on-insulatorstructure comprising a silicon substrate 306, a single-crystal-siliconlayer 304 and an insulator layer 305, such as an oxide layer,therebetween. The single-crystal-silicon layer 304 is etched, forexample using a dry etch, to define the movable part 332 and thestationary parts 322 and 342 of the switch 330 as shown in FIGS. 11 and12. Further, a through hole 360 is defined in the single-crystal-siliconlayer 304 to accommodate the stop block 350 shown in FIG. 16.

Next, as shown in FIGS. 13 and 14, the single-crystal-silicon layer 304is etched, for example using a dry etch, to form the plurality ofwaveguides 334, 324 and 344 in the movable part 332 and the stationaryparts 322 and 342, respectively. Then, as shown in FIGS. 15 and 16, asacrificial layer of material 362, such as an oxide, is deposited andpatterned to form one or more anchor holes 364 in the silicon substrate306 and/or the single-crystal-silicon layer 304. As shown in FIGS. 15and 16, the anchor hole 364 formed in the silicon substrate is for thestop block 350 and the anchor holes 364 formed in thesingle-crystal-silicon layer 304 are for the bumpers 352, when included.The stop block 350 and the bumpers 352 are formed by depositing a layerof structural material 354, for example polysilicon, and patterning thelayer of structural material 354.

The sacrificial layer 362 and at least part of the insulator layer 305are removed by a release etch, such as a wet etch, to obtain the switch330 shown in FIGS. 17 and 18.

FIGS. 19-24 show a second exemplary embodiment of a self-alignedwaveguide switch 430 for optical fiber communication that may be used inthe optical add/drop multiplexer of this invention. As shown in FIG. 19,the switch 430 includes a movable part 432 with a plurality ofwaveguides 434. A stationary input part 422 of the switch 430 is inoptical communication with, for example, the demultiplexer of theoptical add/drop multiplexer and has a plurality of waveguides 424. Astationary output part 442 is in optical communication with, forexample, the multiplexer of the optical add/drop multiplexer and has aplurality of waveguides 444.

As shown in FIG. 20, a stop block 450 is anchored to a substrate 403 of,for example, the optical add/drop multiplexer. According to thisembodiment, a cutout section or window 452 is formed in the movable part432. The window 452 may be formed such that a section 454 of the layerused to fabricate the movable part 432 is connected to the stop block450, as shown in FIG. 20.

As above, a set of offsets d1 and d2 is defined between the waveguides434 of the movable part 432 and the waveguides 424 and 444 of thestationary parts 422 and 442. As described further below, the set ofoffsets d1 and d2 is defined by photolithography before the movable part432 is released from the substrate 403. The stop block 450 and window452 are used to control the position of the movable part 432 of theswitch 430 by limiting the movement of the movable part 432. The sameoffsets d1 and d2 are used to define the edges of the window 452 and/orsection 454 such that the distance from the stop block 450 or section454 to one inside edge of the window 452 is d1 and the distance from thestop block 350 or section 454 to one inside edge of the window 452 isd2.

This arrangement provides two stable positions for the movable part 432of the switch 430. As shown in FIGS. 21 and 22, the movable part 432moves to the left in direction of the arrow A until the stop block 450or section 454 contacts one inside edge of the window 452. In thisposition, the left waveguide 434 of the movable part 432 is aligned withthe left waveguides 424 and 444 of the stationary parts 422 and 442. Asshown in FIGS. 23 and 24, the movable part 432 moves to the right indirection of the arrow B until the stop block 450 or section 454contacts the other inside edge of the window 452. In this position, theleft waveguide 434 of the movable part 432 is aligned with the rightwaveguides 424 and 444 of the stationary parts 422 and 442.

FIGS. 25-26 show a modification of the second exemplary embodiment ofthe self-aligned waveguide switch 430. This modification utilizes foursets of stop blocks 450 and windows 452 which may provide more stabilityand reliability for the switch 430.

According to this invention, the set of offsets d1 and d2 is defined ina lithographic process on one masking layer so that the set may be veryaccurately controlled. In other words, the structures that align thewaveguides of the switch are determined by the geometrical dimensions d1and d2 in the same structural layer. The avoids the disadvantages ofalignment between different structural layers. A more detaileddescription of a unique silicon-on-insulator based micromachiningprocess according to this invention is described with reference to FIGS.27-57. The process is described below in conjunction with thefabrication of a micro-mechanical actuator for moving the switch and aV-groove for optical fiber connection. However, the actuator and/or theconnection may or may not be fabricated C with the switch. Thus, itshould be understood that the design and configuration of the actuatorand/or the connection of the optical fiber are illustrative and notlimiting. The V-groove fabrication and alignment of optical fibers withthe add/drop multiplexer of this invention is described in more detailin copending U.S. Pat. No. 6,510,275, filed herewith and incorporated byreference in its entirety.

In general, polysilicon surface micromachining uses planar fabricationprocess steps common to the integrated circuit (IC) fabrication industryto manufacture microelectromechanical or micromechanical devices. Thestandard building-block process consists of depositing andphotolithographically patterning alternating layers on a substrate. Thealternating layers consist of low-stress polycrystalline silicon (alsotermed polysilicon) and a sacrificial material such as silicon dioxideon a substrate. Vias etched through the sacrificial layers provideanchor points to the substrate and between the polysilicon layers. Thepolysilicon layers are patterned to form mechanical elements of themicromachined device. The mechanical elements are thus formedlayer-by-layer in a series of deposition and patterning process steps.The silicon dioxide layers are then removed by exposure to a selectiveetchant, such as hydrofluoric acid (HF), which does not attack thepolysilicon layers. This releases the mechanical elements formed in thepolysilicon layers for movement thereof.

As shown in FIG. 27, the exemplary embodiment begins with asilicon-on-insulator wafer 400 comprising a silicon substrate 402, asingle-crystal-silicon layer 404 and an insulator layer 406, such as anoxide layer, therebetween.

As shown in FIG. 28, a first mask layer 410, such as an oxide, isdeposited, for example by low pressure chemical vapor deposition(LPCVD), onto the single-crystal-silicon layer 404 and onto the siliconsubstrate 402. The first mask layer 410 may be, for example,approximately 1.0 micron thick. The first mask layer 410 serves as amasking layer for protecting the single-crystal-silicon layer 404 duringa subsequent etch of the silicon substrate 402. As shown in FIG. 29, ahole 414 is patterned in the first mask layer 410 to define an openingfor the subsequent etch.

The silicon substrate 402 is then etched, for example in a KOH solution,to create a triangular or trapezoidal hole 416 in the silicon substrate402, as shown in FIG. 30. An edge of the hole 416 is used as a referencefor subsequent photolithographic steps of the process that requireprecise alignment to the <110>direction of the silicon substrate 402. Asshown in FIG. 31, the first mask layer 410 is then removed, for example,using a wet etch.

A second mask layer 420, such as an oxide, is deposited, for example bylow pressure chemical vapor deposition (LPCVD), onto the etched siliconsubstrate 402 and onto the single-crystal-silicon layer 404, as shown inFIG. 32. The second mask layer 420 may be, for example, approximately0.25 micron thick. The second mask layer 420 serves to protect theetched silicon substrate 402 during a subsequent etch of thesingle-crystal-silicon layer 404.

The second mask layer 420 is then patterned, for example using aphotoresist (not shown). As shown in FIGS. 33 and 34, thesingle-crystal-silicon layer 404 is etched, for example using a dry etchsuch as a reactive ion etch, with the photoresist and/or the second masklayer 420 as masking layers. As shown, the etching may over-etchapproximately 0.15 microns into the insulator layer 406.

In order to improve the quality of the structures in thesingle-crystal-silicon layer 404, a dry oxidation may be performed togrow a thin oxide 422, for example approximately 1000 Angstroms thick,on sidewalls 424, as shown in FIG. 35. As shown in FIG. 36, the thinoxide 422 is then removed, for example, using a wet etch such as abuffered HF etch for 2 minutes. This wet etch will also removeapproximately 2000 additional Angstroms of the insulator layer 406.

A third mask layer (not shown), such as an oxide, is deposited, forexample by low pressure chemical vapor deposition (LPCVD), onto theetched single-crystal-silicon layer 404. As shown in FIG. 37, anchorholes 436 are etched, for example using a wet etch, to remove theinsulator layer 406.

A nitride layer 440 is then deposited, for example by low pressurechemical vapor deposition (LPCVD), as shown in FIG. 38. The nitridelayer 440 provides an anti-reflection coating for the waveguides of theswitch and also serves as a masking layer for a subsequent etch of aV-groove.

A fourth mask layer (not shown), such as a photoresist, is deposited andpatterned over the nitride layer 440. The patterned photoresist is usedto define ridge waveguides and an opening for a V-groove, as shown inFIG. 39, whereby exposed portions of the nitride layer 440 and the thirdmask layer 430 and a thin portion, about 500 Angstroms, of the insulatorlayer 406 are etched away.

A photoresist (not shown) along with the remaining nitride layer 440 andthe remaining third mask layer 430 are used as a mask to define trenchesin the single-crystal-silicon layer 304 that form ridge waveguides 442,as shown in FIG. 40, in conjunction with a dry etch, such as a reactiveion etch. Because the insulator layer 406 is much thicker than the thirdmask layer 430, a layer of about 4000 Angstroms of the insulator layer406 will remain after the reactive ion etch. Thus, the silicon substrate402 is not attacked by the reactive ion etch.

In order to improve the quality of the ridge waveguides 442 in thesingle-crystal-silicon layer 404, a dry oxidation may be performed togrow another thin oxide 444, for example approximately 1000 Angstromsthick, on sidewalls 446, as shown in FIG. 41. As above, the thin oxide444 is then removed, for example, using a wet etch such as a buffered HFetch for 2 minutes.

Next, as shown in FIG. 42, a fifth mask layer 450, such as an oxide, isdeposited, for example by low pressure chemical vapor deposition(LPCVD), as a mask for a subsequent wet etch. The fifth mask layer 450may be approximately 5000 Angstroms thick. The fifth mask layer 450 ispatterned, for example using a photoresist, as shown in FIG. 43. Thefifth mask layer 450 serves as a mask for removing the nitride layer 440with a wet etch, for example, in phosphoric acid, as shown in FIG. 44.In particular, this wet etch removes the nitride layer 440 from theridge waveguides 442 to avoid increases in optical loss from curling ofthe nitride layer 440.

A sixth mask layer 460, such as a 0.3 micron LPCVD deposited undopedoxide layer and a 1.7 micron sacrificial phosphosilicate-glass layer, isformed, as shown in FIG. 45. The undoped oxide layer helps preventdoping of the ridge waveguides 442 during subsequent high temperatureannealing. The sixth mask layer 460 is patterned using aphotolithographic process so that anchor holes 462 are defined andopened during a wet etch, as shown in FIG. 46. Then, as shown in FIG.47, a photoresist (not shown) is used as a mask to define vias that areopened by a dry etch, such as a reactive ion etch.

A layer of structural material 470, such as polysilicon, is thendeposited, doped and annealed, as shown in FIG. 48. The layer ofstructural material 470 may be, for example, 3 microns thick. Using oneor more suitable mask layers (not shown), microstructures such as ananchor stop 472, a bumper 474 and/or a bridge 476 may be shaped in thelayer of structural material 470 by one or more etches. For example, onemask may be used with a dry etch to cut through the layer of structuralmaterial 470, while another mask may be used with another etch to cutthrough the layer of structural material 470 and thesingle-crystal-silicon layer 404, as shown in FIGS. 49 and 50,respectively. Using two etching steps will help to minimize undesirablelateral etch on the microstructures formed in the layer of structuralmaterial 470. For example, the microstructures such as the anchor stop472, the bumper 474 and/or the bridge 476 may be fabricated with highaccuracy. Using a single etch to cut through layers of differentthickness may result in an over-etch on the thinner areas.

As shown in FIG. 51, a layer of protective material 480, such as siliconnitride, is deposited as a mask to protect the polysilicon andsingle-crystal silicon microstructures from a subsequent etch. The layerof protective material 480 is patterned using a mask (not shown), suchas a photoresist, and selectively removed, for example using a dry etch,as shown in FIG. 52. Then, the sixth mask layer 460 and the remaininginsulator layer 406 are removed using a wet etch to expose the siliconsubstrate 402 where a V-groove is to be formed, as shown in FIG. 53. Itshould be noted that the alignment of the photoresist is not criticalbecause the opening for the V-groove is primarily defined by the nitridelayer 440 already patterned.

As shown in FIG. 54, a V-groove 482 is etched into the silicon substrate402, for example, using a KOH etch. After the KOH etch, the layer ofprotective material 480 is removed, as shown in FIG. 55, using a wetetch, for example in phosphoric acid.

A thick photoresist (not shown) is then deposited and patterned using alithographic process to form a mask. The mask defines one or morebonding pads 484, as shown in FIG. 56, that are formed, for example,with gold using a sputtering and lift-off process. Finally, a wet etch,for example in hydrofluoric acid, is used to release themicrostructures, as shown in FIG. 57.

In a modification of this fabrication process, one of the mask layersmay be eliminated to reduce the cost and time required for the process.This modification follows the previous process through the removal ofthe thin oxide 422 using a wet etch as shown in FIG. 36.

A third mask layer 530 is deposited, for example by low pressurechemical vapor deposition (LPCVD), onto the etchedsingle-crystal-silicon layer 504 to define anchor holes 536. In thiscase, the third mask layer 530 is a nitride layer, as shown in FIG. 58.The nitride layer provides an anti-reflection coating for the waveguidesof the switch and also serves as a masking layer for a subsequent etchof a V-groove. The third mask layer 530 also is used to define ridgewaveguides and an opening for a V-groove, as shown in FIG. 59, wherebyexposed portions of the third mask layer 530 and a thin portion, about500 Angstroms, of the insulator layer 506 are etched away.

A photoresist (not shown) along with the remaining third mask layer 530are used as a mask to define trenches in the single-crystal-siliconlayer 504 that form ridge waveguides 542, as shown in FIG. 60, inconjunction with a dry etch, such as a reactive ion etch. Because theinsulator layer 506 is much thicker than the third mask layer 530, alayer of about 4000 Angstroms of the insulator layer 506 will remainafter the reactive ion etch. Thus, the silicon substrate 502 is notattacked by the reactive ion etch.

In order to improve the quality of the ridge waveguides 542 in thesingle-crystal-silicon layer 504, a dry oxidation may be performed togrow another thin oxide 544, for example approximately 1000 Angstromsthick, on sidewalls 546, as shown in FIG. 61. As above, the thin oxide544 is then removed, for example, using a wet etch such as a buffered HFetch for 2 minutes.

Next, as shown in FIG. 62, a fourth mask layer 550, such as an oxide, isdeposited, for example by low pressure chemical vapor deposition(LPCVD), as a mask for a subsequent wet etch. The fourth mask layer 550may be approximately 5000 Angstroms thick. The fourth mask layer 550 ispatterned, for example using a photoresist, as shown in FIG. 63. Thefourth mask layer 550 serves as a mask for removing the nitride layer530 with a wet etch, for example, in phosphoric acid, as shown in FIG.64. In particular, this wet etch removes the nitride layer 530 from theridge waveguides 542 to avoid increases in optical loss from curling ofthe nitride layer 530.

A fifth mask layer 560, such as a 0.3 micron LPCVD deposited undopedoxide layer and a 1.7 micron sacrificial phosphosilicate-glass layer, isformed, as shown in FIG. 65. The undoped oxide layer helps preventdoping of the ridge waveguides 542 during subsequent high temperatureannealing. The fifth mask layer 560 is patterned using aphotolithographic process so that anchor holes 562 and vias 564 aredefined and opened during a wet etch, as shown in FIG. 66. Then, asshown in FIG. 67, a layer of insulating material 566, such as a nitride,is deposited to provide insulation in the anchor holes 562 and/or thevias 564. The layer of insulating material 566 is subsequently patternedusing a dry etch, as shown in FIG. 68. If necessary, an oxide layer (notshown) may be deposited on the layer of insulating material 566 for wetetching. The modified process then proceeds as described above withrespect to FIGS. 48-57.

While this invention has been described in conjunction with variousexemplary embodiments, it is to be understood that many alternatives,modifications and variations would be apparent to those skilled in theart. Accordingly, Applicants intend to embrace all such alternatives,modifications and variations that follow in the spirit and scope of thisinvention.

For example, modifications such as those described in copending U.S.patent application Ser. No. 09/683,533, which is incorporated byreference in its entirety, are contemplated. Also, while techniquesdescribed above for fabricating the silicon demultiplexer, the siliconoptical switches and the silicon multiplexer are particularly suitable,it should be understood that any known or later developed processingtechnique for silicon structures may be used. For example, conventionalphotolithography and etching techniques may be used.

What is claimed is:
 1. A monolithic reconfigurable optical multiplexer,comprising: a silicon demultiplexer; a plurality of silicon opticalswitches; and a silicon multiplexer, wherein the silicon demultiplexer,optical switches and multiplexer are monolithically integrated on asingle silicon chip, wherein the single silicon chip comprises a siliconon insulator wafer and the silicon demultiplexer, the plurality ofsilicon optical switches and the silicon multiplexer are fabricated in asingle crystal silicon device layer of the wafer.
 2. The multiplexer ofclaim 1, wherein the silicon demultiplexer and the silicon multiplexereach comprise a diffraction grating.
 3. The multiplexer of claim 1,wherein the silicon demultiplexer and the silicon multiplexer eachcomprise an arrayed waveguide grating.
 4. The multiplexer of claim 1,wherein the silicon optical switches comprise latching switches.
 5. Themultiplexer of claim 1, wherein the silicon optical switches comprise1×2 optical switches.
 6. The multiplexer of claim 1, wherein the siliconoptical switches comprise 2×2 optical switches.
 7. The multiplexer ofclaim 1, wherein the silicon optical switches comprise waveguideswitches.
 8. The multiplexer of claim 1, wherein the silicon opticalswitches comprise micromachined torsion mirrors.
 9. The multiplexer ofclaim 1, wherein the silicon optical switches comprise at least one ofelectrostatic, magnetostatic, piezoelectric and thermal micromirrors.10. The multiplexer of claim 1, wherein the silicon optical switchescomprise tilting micromirrors.
 11. A method for modifying an opticalsignal, comprising inputting an optical signal comprising a data streaminto a monolithic reconfigurable optical multiplexer including at leastone silicon demultiplexer, a plurality of silicon optical switches andat least one silicon multiplexer monolithically integrated on a singlesilicon chip, wherein the single silicon chip comprises a silicon oninsulator wafer with the silicon demuitiplexer, the plurality of siliconoptical switches and the silicon multiplexer fabricated in a singlecrystal silicon device layer of the wafer.
 12. The method of claim 11,further comprising outputting an optical signal comprising a modifieddata stream.
 13. A method for manufacturing a monolithic reconfigurableoptical multiplexer, comprising fabricating at least one silicondemultiplexer, a plurality of silicon optical switches and at least onesilicon multiplexer in a single silicon layer, wherein the singlesilicon layer comprises a single crystal silicon device layer of asilicon on insulator wafer.
 14. The method of claim 13, wherein thefabricating of the at least one demultiplexer and the at least onemultiplexer comprises fabricating diffraction gratings.
 15. The methodof claim 13, wherein the fabricating of the at least one demultiplexerand the at least one multiplexer comprises fabricating arrayed waveguidegratings.
 16. An optical communications system, comprising: an inputoptical fiber; the monolithic reconfigurable optical multiplexer ofclaim 1, the silicon demultiplexer communicating with the input opticalfiber, the plurality of silicon optical switches communicating betweenthe silicon demultiplexer and the silicon multiplexer; and an outputoptical fiber communicating with the silicon multiplexer.